The process of electrical discharge machining (EDM) is well known. In the field of traveling wire EDM, an electrical potential (voltage) is established between a continuously moving EDM wire electrode and an electrically conductive workpiece. The potential is raised to a level at which a discharge is created between the EDM wire electrode and the workpiece. The intense heat generated by the discharge will melt and/or vaporize a portion of both the workpiece and the wire to thereby remove, in a very small increment, a piece of the workpiece. By generating a large number of such discharges a large number of increments are removed from the workpiece whereby the workpiece can be cut very exactly to have a desired planar contour. A dielectric fluid is used to establish the necessary electrical conditions to initiate the discharge and to flush debris from the active machining area.
The residue resulting from the melting and/or vaporization of a small increment (volume) of the surface of both the workpiece and the EDM wire electrode is contained in a gaseous envelope (plasma). The plasma eventually collapses under the pressure of the dielectric fluid. The liquid and the vapor phases created by the melting and/or vaporization of material are quenched by the dielectric fluid to form solid debris. The cutting process therefore involves repeatedly forming a plasma and quenching that plasma. This process will happen sequentially at nanosecond intervals at many spots long the length of the EDM wire.
It is important for flushing to be efficient because, if flushing is inefficient, conductive particles build up in the gap which can create the potential for electrical arcs. Arcs are very undesirable as they cause the transfer of a large amount of energy that causes large gouges or craters, i.e. metallurgical flaws, to be introduced into the workpiece and the EDM wire electrode. Such flaws in the wire could cause the EDM wire to break catastrophically.
An EDM wire must possess a tensile strength that exceeds a desired threshold value to avoid tensile failure of the wire electrode induced by the preload tension that is applied, and should also possess a high fracture toughness to avoid catastrophic failure induced by the flaws caused by the discharge process. Fracture toughness is a measure of the resistance of a material to flaws which may be introduced into the material and which can potentially grow to the critical size that could cause catastrophic failure of the material. The desired threshold tensile strength for an EDM wire electrode is thought to be in the range 60,000 to 90,000 psi (414 to 620 N/mm2).
It is known in the prior art to use an EDM wire electrode with a core composed of a material having a relatively high mechanical strength with a relatively thin metallic coating covering the core and comprising at least 50% of a metal having a low volumetric heat of sublimation such as zinc, cadmium, tin, lead, antimony, bismuth or an alloy thereof. Such a structure is disclosed is U.S. Pat. No. 4,287,404 which discloses a wire having a steel core with a coating of copper or silver which is then plated with a coating of zinc or other suitable metal having a low volumetric heat of sublimation.
It is also known from the prior art, for instance from U.S. Pat. No. 4,686,153, to coat a copper clad steel wire with zinc and thereafter to heat the zinc coated wire to cause inter-diffusion between the copper and zinc to thereby convert the zinc layer into a copper zinc alloy. That patent describes the desirability of a beta phase alloy layer for EDM purposes. The copper zinc has a concentration of zinc of about 45% by weight with the concentration of zinc decreasing radially inward from the outer surface. The average concentration of zinc in the copper zinc layer is less than 50% by weight but not less than 10% by weight. The surface layer therefore includes beta phase copper-zinc alloy material at the outer surface since beta phase copper zinc alloy material has a concentration of zinc ranging between 40%-50% by weight. While this patent recognized that a copper-zinc alloy layer formed by means of a diffusion anneal process could potentially contain epsilon phase (approximately 80% zinc content), gamma phase (approximately 65% zinc content), beta phase (approximately 45% zinc content), and alpha phase (approximately 35% zinc content), the patent asserted that the preferred alloy material is beta phase in the coating.
Others in the prior art, for instance U.S. Pat. No. 5,762,726, recognized that the higher zinc content phases in the copper-zinc system, specifically gamma phase, would be more desirable for EDM wire electrodes, but the inability to cope with the brittleness of these phases limited the commercial feasibility of manufacturing such wire.
This situation changed with the technology disclosed in U.S. Pat. No. 5,945,010. By employing low temperature diffusion anneals, the inventor was able to incorporate brittle gamma phase particles in a coating on various copper containing metallic substrates. However due to the brittle characteristics of the gamma phase brass alloy, the resultant microstructure is characterized by a discontinuous coating where the substrate material is exposed to the gap at these discontinuities. Therefore, the inferior cutting properties of the substrate, as compared to gamma phase brass alloy will retard the overall performance of the wire in proportion to the amount of substrate area exposed and the relative cutting performance of the gamma phase alloy and the exposed substrate material. The '010 patent found epsilon phase to be too unstable to be incorporated in the resultant high zinc content alloy coating, although the potential for a brittle epsilon coating was acknowledged.
There have been a number of attempts to combine the benefits of several coating materials by constructing multi-layered coatings. Examples of these attempts can be found in U.S. Pat. Nos. 6,306,523, 6,566,622 and U.S. Pat. No. 6,781,081. All of the multi-layered constructions disclosed in these patents containing gamma phase brass are subject to the same limitation that a single layered gamma phase brass suffers from, the brittleness of the gamma phase creates a discontinuous layer that is exposed to the workpiece, even when the gamma phase is covered by an additional layer.